The invention relates to cavitation enhanced liquid atomization. More particularly, the invention relates to atomizing a fluid comprising a solution of the liquid to be atomized and a lower boiling cavitation liquid, by contacting the fluid under pressure and while flowing, with a pressure reducing means to reduce the fluid pressure and thereby produce nucleation and growth of bubbles comprising vapor of the cavitation liquid in the fluid, at a temperature below the bubble point of the solution, and then passing the fluid through an atomizing means into a lower pressure atomizing zone. Bubble nucleation is induced upstream of the atomizing means. This is useful for atomizing a hot FCC feed oil into a catalytic cracking reaction zone, using a lower boiling hydrocarbon as the cavitation liquid.
Fluid atomization is well known and used in a wide variety of applications and processes. These include, for example, aerosol sprays, the application of pesticides and coatings, spray drying, humidification, mixing, air conditioning, and chemical and petroleum refinery processes. For most applications, a fluid under pressure, with or without the assistance of an atomizing gas, is forced through a pressure reducing orifice in an atomization nozzle. Atomization occurs as the fluid passes through the orifice and into the lower pressure zone downstream. The degree of atomization is determined by the orifice size, the pressure drop across the orifice, fluid density, viscosity, and surface tension, etc., as is known. Atomization is increased and the droplet size is decreased, with decreasing orifice size and increasing pressure drop. Atomizing relatively viscous fluids at high flow rates, such as the heavy petroleum oil feeds used in fluidized catalytic cracking (FCC) processes, or fluid cat cracking as it is also called, is particularly challenging. FCC is an established and widely used process in the petroleum refining industry, primarily for converting high boiling petroleum oils to more valuable lower boiling products, including gasoline and middle distillates such as kerosene, jet and diesel fuel, and heating oil. In an FCC process, the preheated oil feed is mixed with steam or a low molecular weight (e.g., C4xe2x88x92) gas under pressure, to form a two phase, gas and liquid fluid. This fluid is passed through a pressure-reducing orifice into a lower pressure atomization zone, in which the gas expands and the oil is atomized, and brought into contact with a particulate, hot cracking catalyst. The atomization is effected primarily by the shearing action between the gas and liquid phases, as the fluid passes through the orifice and into the lower pressure atomization zone. The FCC riser comprises both the feed atomization zone and the cat cracking zone. Steam is more often used than a light hydrocarbon gas, to reduce the vapor loading on the on the gas compression facilities and the downstream products fractionation. However, the use of steam produces sour water, which enhances corrosion. Sour water is also environmentally unfriendly and must therefore be treated before disposal. There is a need therefore, for a process that either reduces or eliminates the amount of steam or low molecular weight gas atomizing agents.
The invention relates to a liquid atomizing process in which a fluid comprising a solution of the liquid to be atomized and a lower boiling cavitation liquid, is contacted under pressure and while flowing, with a pressure reducing means to reduce the fluid pressure and thereby produce nucleation of bubbles comprising the cavitation liquid vapor in the fluid, at a temperature below the bubble point of the solution, and then passing the fluid through an atomizing means into a lower pressure atomizing zone. Thus, the cavitation bubbles comprise vapor of the cavitation liquid. Nucleation of the cavitation bubbles produces a two-phase fluid comprising the vapor bubbles and the liquid solution. While bubble nucleation is produced upstream of the atomizing means, typically and preferably both nucleation and growth of the cavitation bubbles will occur upstream of the atomizing means. Growth of the cavitation bubbles is produced by one or more pressure reducing means and also by the pressure drop in the fluid, as it flows downstream to the atomizing means. A continued pressure drop, even if only slight, assists in stabilizing the bubbles. Additional growth of the cavitation bubbles occurs as the fluid passes through the atomizing means and into the lower pressure atomizing zone, in which it rapidly vaporizes. Passing the so-formed two-phase fluid through the atomizing means also produces shear between the vapor (the cavitation bubbles) and liquid phases, which increases the surface area of the liquid, as reflected in the formation of ligaments, membranes, smaller globules, etc. The atomization produces a spray of liquid droplets into the lower pressure atomizing zone. This is explained in detail below. By pressure is meant a low pressure reducing means, such as one or more static mixers in the fluid line upstream of the atomizing means. By low pressure is meant that the static mixer(s) or other pressure reducing means produces, in the flowing fluid, a pressure drop less than 50 psi, preferably less than 30 psi, more preferably less than 15 psi, and most preferably no greater than 5 psi upstream of the atomizing means, with a typical pressure drop ranging from 1 to 5 psi. Cavitation is a phenomena in which a pressure drop induces bubble formation in a liquid, at a temperature below the bubble point of the liquid. Thus, cavitation occurs by reducing the pressure, while maintaining a constant temperature. This is in contrast to boiling enhanced atomization, in which bubble nucleation is induced by increasing the temperature of the fluid above the bubble point, while maintaining the pressure constant. The cavitation liquid is soluble in the liquid to be atomized at the process conditions and either has a lower boiling point than the liquid to be atomized or contains sufficient material boiling below the boiling range of the liquid to be atomized, to form bubbles which grow and expand for the atomization. Typically this means that at least 0.5 wt. %, preferably at least 1.0 wt. %, and more preferably greater than 1.0 wt. %, of the flowing fluid or liquid which comprises liquid to be atomized and cavitating liquid will be vaporized during the initial bubble formation and subsequent atomization of the flowing liquid to be atomized. The cavitating liquid is from 0.1 to 10 wt. %, preferably 0.3 to 5.0 wt. %, more preferably 0.5 to 2.0 wt. %, based on flowing liquid to be atomized. From 1 to 100% of the cavitating liquid can be vaporized depending on the process conditions. Thus, the flowing fluid or liquid produced by mixing the cavitating liquid with the liquid to be atomized is preferably a single phase liquid mixture or solution, as opposed to two liquid phases or an emulsion.
The process of the invention is useful for atomizing a wide variety of liquids, including chemical and refinery process liquids, such as atomizing a hot FCC feed oil into a cat cracking reaction zone, using a lower boiling hydrocarbon as the cavitation liquid. In, for example, an FCC process, a two-phase mixture of an FFC oil feed liquid and an atomizing agent comprising steam flows through a feed injector which terminates at its downstream end in an atomizing means comprising an atomizing orifice. The downstream side of the atomizing orifice opens downstream into a spray distributor, as is known. In the practice of the invention, a cavitating fluid, comprising one or more lower boiling hydrocarbons or lower boiling hydrocarbon fractions, is mixed with the hot oil either upstream of the injector or within the injector, to form the fluid solution which, at this point, is a liquid. The injector typically comprises one or more conduits for flowing one or more liquids through and terminates at its downstream end in an atomizing means. The liquid solution of FCC feed oil and the one or more cavitating liquids is maintained at a pressure and temperature, such that cavitation preferably does not occur until the flowing fluid contacts one or more pressure reducing means in the injector, to produce a pressure drop in the fluid and thereby induce nucleation and growth of bubbles comprising the vapor of the cavitating fluid(s) dissolved in the hot oil. This bubble nucleating pressure drop may be as much as one-third of the pressure drop of the fluid through the injector and into the FCC cat cracking zone, as an oil spray comprising droplets of the atomized oil. The pressure drop inducing means is located upstream of the atomizing means. Such means will preferably include one or more static mixing means located in the fluid conduit upstream of the atomizing means. In one embodiment, a plurality of such means may be located in the fluid conduit so that the flowing fluid successively contacts more than one such means as it flows downstream to the atomizing means. This embodiment will produce bubble nucleation and growth in the oil feed as it approaches the atomizing means. The fluid pressure upstream of the pressure drop means is preferably maintained sufficiently high to prevent bubble nucleation and this means a pressure greater than the vapor pressure or bubble nucleation pressure of the solution at the design temperature. The pressure in the atomizing zone is greater than the vapor pressure of the liquid to be atomized, but lower than the vapor pressure of the cavitating fluid and preferably sufficiently lower to further promote and ensure rapid vaporization or flashing of the cavitating liquid to assist in forming the spray of liquid droplets. At any given temperature, the greater the pressure differential between the pressure in the atomizing zone and the vapor pressure of the cavitating fluid in the atomizing zone, the more rapid and violent will be its expansion, which translates into a smaller average droplet size of the atomized liquid. The atomizing orifice may comprise the upstream entrance of a controlled expansion atomizing zone, such as the fan-shaped distributor of the type disclosed in U.S. Pat. No. 5,173,175 which provides a fan-shaped spray of the atomized liquid into the FCC cat cracking reaction zone. The orifice may also comprise a shaped slot at the end of a conduit, for providing a more or less fan-shaped spray as disclosed, for example, in U.S. Pat. Nos. 4,784,328 and 5,289,976. Other embodiments will be explained in detail below.
The process of the invention is useful for atomizing any liquid, including aqueous liquids as well as hydrocarbonaceous liquids. In the case of water, for example, the cavitating liquid may be acetone, methanol and the like. When used in connection with an FCC cat cracking process, the practice of the invention reduces and preferably eliminates the use of steam for feed atomization and the concomitant sour water production, clean-up and disposal. It also reduces and preferably eliminates the use of a hydrocarbon gas (e.g., C1-C5) to form a two-phase fluid for atomization. In addition, the use of the liquid phase process of the invention eliminates the hydraulic hammering and piping vibration problems associated with conventional gas-liquid phase fluid atomization. In a more detailed embodiment relating to FCC feed atomization, the invention comprises a fluid cat cracking process which comprises the steps of:
(a) contacting a flowing fluid, under pressure and comprising a solution of FCC feed oil and a cavitating liquid which comprises one or more hydrocarbon liquids or fractions containing material which boils below the boiling range of said oil feed, with a pressure drop means to reduce the pressure of said flowing fluid and produce nucleation of bubbles comprising vapor of said cavitating liquid at a temperature below the bubble point of said solution, to form a two-phase fluid comprising said bubbles and liquid solution;
(b) passing said two-phase fluid downstream into and through an atomizing means into an atomizing zone which is at a pressure lower than that of said fluid upstream of said atomizing means, to atomize said fluid and form a spray comprising liquid droplets of said feed oil, wherein said atomizing zone comprises a cat cracking reaction zone, and
(c) contacting said spray with a particulate, hot, regenerated cracking catalyst in said reaction zone at reaction conditions effective to catalytically crack said feed oil and produce lower boiling hydrocarbons.
The lower boiling hydrocarbons produced by the cracking reaction are separated from the spent catalyst particles, in a separation zone, are recovered and then typically sent to further processing, including fractionation. The cracking reaction also produces spent catalyst particles, which contain strippable hydrocarbons and coke, as is known. The spent catalyst particles are stripped in a stripping zone, to remove the strippable hydrocarbons to produce stripped, coked catalyst particles. The stripped, coked catalyst particles are passed into a regeneration zone, in which they are contacted with oxygen, at conditions effective to burn off the coke and produce the hot, regenerated catalyst particles, which are then passed back up into the reaction zone. The reaction zone of an FCC cat cracking process usually comprises a riser and is known as a riser reaction zone.